There is interest in wireless communication systems transmitting signals over a Multiple-Input Multiple-Output (MIMO) channel. In a MIMO transmission scheme, signals are transmitted simultaneously from multiple transmit antennas at the transmitter. Multiple receive antennas in the receiver are used to detect all the transmitted signals. The data stream is divided into multiple sub-streams or layers of data. Each data layer is transmitted independently of the other layers. The aim of MIMO transmission schemes is to offer increased bit rates by transmitting multiple layers of data in parallel.
Different MIMO transmission schemes include a layered space-time architecture for multi-element antenna arrays often named BLAST (Bell-Labs Layered Space-Time architecture), designed for systems with flat fading channels. The BLAST method can be divided into two sub-classes: Diagonal BLAST (D-BLAST) and Vertical BLAST (V-BLAST). In the BLAST MIMO schemes, a stream of data is de-multiplexed into several sub-streams or layers of data, each of which is encoded with an error correcting channel code and interleaved independently of the other layers. These kinds of MIMO schemes, where each layer is encoded separately, are referred to for convenience as “per-layer coding.”
FIG. 1 illustrates a transmitter 10 using a per-layer coding MIMO scheme in which a stream of data packets is received in a demultiplexer 12 and de-multiplexed into sub-layers (four are shown for illustration). Each sub-layer is channel encoded using a selected coding rate in a corresponding channel encoders 14. The coded bits are then mapped in block 16 to particular transmission branches that each include a modulator 18 and an antenna 20. In FIG. 1, the demultiplexing into layers occurs prior to the channel encoder. Another example of a per-layer coding MIMO scheme is a PARC (Per Antenna Rate Control) scheme. The per-layer coding MIMO schemes transmit multiple code blocks in parallel. The interleaving dimension is fixed (e.g. time), and does not depend on the radio conditions.
An alternative approach to per-layer coding MIMO transmission performs the channel coding prior to de-multiplexing onto different layers. This approach is referred to for convenience as “all-layer coding” and is illustrated in FIG. 2. The transmitter 10 in FIG. 2 directs the data stream to the channel encoder 14 which channel encodes the data stream. The coded data stream is demultiplexed into separate coded layers in demultiplexer 12. The remaining operations in the layer processing branches are similar to those described in FIG. 1. For these kinds of MIMO schemes, a fixed interleaving scheme is used, where the encoded data block is interleaved over all antennas, and where only one code block is transmitted at a time. Fixed interleaving does not take radio conditions into account.
In a MIMO receiver, several different algorithms may be used to detect the signals transmitted from the multiple transmit antennas. Detection involves both demodulation and decoding. One example is a multi-staged detection of the transmitted layers. A multi-stage detector in a MIMO receiver 22 is shown in FIG. 3. A suppressor 24 suppresses all layers except one. That one layer is demodulated in demodulator 26 and then decoded in a decoder 28 while the other remaining layers are suppressed. After a layer has been detected (demodulated and decoded), it is cancelled, i.e., subtracted, before the detection of another layer. In the last detection stage, all except one layer has been cancelled in a preceding detection stage. The cancellation of a detected layer improves the detection of the remaining layers. In order to reduce the complexity and processing delay, the receiver may cancel a detected layer directly after the demodulation. In that case, the decoding is done after the cancellation. This approach is particularly useful in a mobile radio communications system such as GSM/EDGE where channel coding is interleaved over several bursts.
In modern wireless networks, e.g., GSM through EGPRS, WCDMA through WCDMA Evolved and cdma2000 through 1XEV, etc., Link Adaptation (LA) is used to adapt the channel coding rate, and possibly also the modulation scheme, to the radio environment. When the channel quality is good, it is possible to transmit more information over the channel then when the channel quality is poor. Evaluating signal quality could, for example, be based on average Signal-to-Noise-Ratio (SNR).
In EGPRS, there are nine Modulation and Coding Schemes (MCS) defined, with code rates of the error correcting channel codes ranging from 0.37 (for MCS-5) to 1.0 (for MCS-4 and MCS-9). Two different modulations are also used: Gaussian Minimum Shift Keying (GMSK) and 8-ary Phase Shift Keying (8-PSK). MCS-1 to MCS-4 use GMSK, and MCS-5 to MCS-9 use 8-PSK modulated signals. Different MCS's are used depending on the detected SNR. For a high SNR, the higher order modulation, 8-PSK, and a high code rate may be used.
GSM is a TDMA (Time Division Multiple Access) based system. In EGPRS, information bits are divided into RLC (Radio Link Control) blocks. Radio blocks are then formed from one or two RLC blocks, and each radio block is transmitted over four bursts (data units). The channel can vary quite significantly between different bursts. Although this variation is beneficial for MCS's with a low code rate, it is usually detrimental for MCS's with a high code rate. The reason for this is that for low rate codes, i.e., a large amount of redundancy, channel variations improve decoder performance. For high rate codes, i.e., a small amount of redundancy, channel variations degrades the decoder performance. To reduce the channel variations for MCS-8 and MCS-9, the radio block is divided into two blocks, where each block includes two consecutive data unit bursts.
For a MIMO transmission scheme to work efficiently, it should be combined with link adaptation (LA). Consider the simplistic examples shown in FIG. 4. A base station (BS) serves a cell area that includes in this example three mobile radio stations (MS1-MS3). The mobile station MS1 is very close to the base station and enjoys excellent radio channel conditions and thus may only need a high coding rate (small amount of redundancy in the channel code) or no coding at all. In contrast, the mobile station MS2 is much farther away near the edge of the cell area and experiences poor radio channel conditions requiring more channel coding, i.e., a lower coding rate. Radio channel conditions also change rapidly depending on interference from other transmissions, fading, and obstacles. The latter factor is represented as a building blocking the direct radio path between mobile station MS3 and the base station. Link adaptation may be extended to include combinations of modulation and channel coding. If channel quality is high, a higher order modulation can be used together with an appropriate amount of channel coding.
One approach to combining MIMO and link adaptation is to select one fixed MIMO scheme while the MCS is varied depending on the radio environment. But a single MIMO scheme is not always best-suited for all radio environments. A better approach is to also select a MIMO scheme based on radio channel conditions. An additional problem is that one MIMO scheme may not be best for each of multiple coding or MCS schemes.
A further problem with adaptation between different MCS's for a fixed MIMO transmission scheme is error propagation in the receiver. For example, the receiver algorithms using a multi-staged detection of the MIMO layers (see FIG. 3) suffer to a greater or lesser degree from propagation of errors from one detected layer to the subsequently detected layers. For low code rate MCS's, this may not be a very large problem since the error correcting channel code can be used to correct a large portion of these errors. But for the high code rate MCS's, an error in one layer is much more likely to cause errors also in the other layers.
Another important factor in the performance of a transmission scheme is the interleaving or diversity scheme used to allocate data units from a data block to one or more antennas. Examples of interleaving schemes include allocating data units from a data block to different antennas (space diversity), allocating data units to different time slots transmitted from the same antenna (time diversity), and allocating data units to different frequencies transmitted from the same or multiple antennas (frequency diversity). For example, an interleaving scheme that maximizes the diversity (variations in the channel quality) within a coding block, usually improves performance for low code rate MCS's, but often degrades performance for high code rate MCS's, and vice versa. In other words, the diversity resulting from transmitting data units at different times over a time-varying channel can be advantageous or disadvantageous depending on the code rate of the channel code.
For example, in EGPRS, MCS-4, MCS-8 and MCS-9 permit transmission at very high code rates (very little coding protection). So poor channel quality for one data transmission burst is very likely to result in one or more errors in that burst after channel decoding in the receiver notwithstanding a situation where the other bursts are decoded without any detected errors. The poor quality might be the result of a fading dip for the channel, interference from another user in the system, or from high correlation between the received signals for that particular channel realization. The other lower code rate MCS's, however, benefit from large variations in signal quality between the bursts due to a potentially higher average quality of the received code block (RLC block). It is therefore difficult to find one interleaving scheme suitable for multiple MCS's with high and low code rates
Like these time domain variations across different TTIs, quality variations can also be found across different transmit antennas. The correlation of the channels between transmit and receive antennas affects the signal quality in the space or antenna domain. While the quality variation or diversity in the time domain depends on Doppler, frequency hopping, interference, etc., the diversity in the space or antenna domain depends on antenna placement, the radio signal propagation environment, and the receiver architecture. Simply having one the interleaving or diversity scheme or selecting one particular interleaving scheme without regard to the channel coding scheme and the radio channel conditions leads to poor performance and even unacceptable error rates.